Here's why:

* Inertia is the tendency of an object to resist changes in its motion. The more massive an object is, the more inertia it has.

* A balloon is very light and has very little mass. This means it will be the easiest to get moving and the easiest to stop.

Let's compare:

* Bowling ball: Very heavy and therefore has a lot of inertia.

* Horse: Heavy and large, with a lot of inertia.

* Cat: Relatively light, but still heavier than a balloon.

* Balloon: Very light, making it the object with the least inertia.